mm: delete __ClearPageWaiters()

The PG_waiters bit is not included in PAGE_FLAGS_CHECK_AT_FREE, and
vmscan.c's free_unref_page_list() callers rely on that not to generate
bad_page() alerts. So __page_cache_release(), put_pages_list() and
release_pages() (and presumably copy-and-pasted free_zone_device_page())
are redundant and misleading to make a special point of clearing it (as
the "__" implies, it could only safely be used on the freeing path).

Delete __ClearPageWaiters(). Remark on this in one of the "possible"
comments in folio_wake_bit(), and delete the superfluous comments.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Hugh Dickins <[e-mail]>
Tested-by: Yu Zhao <[e-mail]>
Reviewed-by: Yang Shi <[e-mail]>
Reviewed-by: David Hildenbrand <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Nicholas Piggin <[e-mail]>
Cc: Yu Zhao <[e-mail]>
Cc: Michal Hocko <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/page-flags.h

···
 	TESTSETFLAG_FALSE(uname, lname) TESTCLEARFLAG_FALSE(uname, lname)
 
 __PAGEFLAG(Locked, locked, PF_NO_TAIL)
-PAGEFLAG(Waiters, waiters, PF_ONLY_HEAD) __CLEARPAGEFLAG(Waiters, waiters, PF_ONLY_HEAD)
+PAGEFLAG(Waiters, waiters, PF_ONLY_HEAD)
 PAGEFLAG(Error, error, PF_NO_TAIL) TESTCLEARFLAG(Error, error, PF_NO_TAIL)
 PAGEFLAG(Referenced, referenced, PF_HEAD)
 	TESTCLEARFLAG(Referenced, referenced, PF_HEAD)

mm/filemap.c

···
 	}
 
 	/*
-	 * It is possible for other pages to have collided on the waitqueue
-	 * hash, so in that case check for a page match. That prevents a long-
-	 * term waiter
+	 * It's possible to miss clearing waiters here, when we woke our page
+	 * waiters, but the hashed waitqueue has waiters for other pages on it.
+	 * That's okay, it's a rare case. The next waker will clear it.
 	 *
-	 * It is still possible to miss a case here, when we woke page waiters
-	 * and removed them from the waitqueue, but there are still other
-	 * page waiters.
+	 * Note that, depending on the page pool (buddy, hugetlb, ZONE_DEVICE,
+	 * other), the flag may be cleared in the course of freeing the page;
+	 * but that is not required for correctness.
 	 */
-	if (!waitqueue_active(q) || !key.page_match) {
+	if (!waitqueue_active(q) || !key.page_match)
 		folio_clear_waiters(folio);
-		/*
-		 * It's possible to miss clearing Waiters here, when we woke
-		 * our page waiters, but the hashed waitqueue has waiters for
-		 * other pages on it.
-		 *
-		 * That's okay, it's a rare case. The next waker will clear it.
-		 */
-	}
+
 	spin_unlock_irqrestore(&q->lock, flags);
 }
 

mm/memremap.c

···
 	if (WARN_ON_ONCE(!page->pgmap->ops || !page->pgmap->ops->page_free))
 		return;
 
-	__ClearPageWaiters(page);
-
 	mem_cgroup_uncharge(page_folio(page));
 
 	/*

mm/swap.c

···
 		mod_zone_page_state(page_zone(page), NR_MLOCK, -nr_pages);
 		count_vm_events(UNEVICTABLE_PGCLEARED, nr_pages);
 	}
-	__ClearPageWaiters(page);
 }
 
 static void __put_single_page(struct page *page)
···
 			continue;
 		}
 		/* Cannot be PageLRU because it's passed to us using the lru */
-		__ClearPageWaiters(page);
 	}
 
 	free_unref_page_list(pages);
···
 			dec_zone_page_state(page, NR_MLOCK);
 			count_vm_event(UNEVICTABLE_PGCLEARED);
 		}
-
-		__ClearPageWaiters(page);
 
 		list_add(&page->lru, &pages_to_free);
 	}